According to a study at the University of California, a small portion of the population has a form of synaesthesia that results in a spatial perception of time.

"In general, these individuals perceive months of the year in circular shapes, usually just as an image inside their mind's eye," says David Brang of the department of psychology at the University of California, San Diego.

"These calendars occur in almost any possible shape, and many of the synaesthetes actually experience the calendar projected out into the real world."